The problem of aerodynamic surfaces icing is well known in the aviation industry. The term “icing” is used to designate the more or less rapid build-up of a deposit of ice on certain portions of an aircraft (leading edges of blades, propellers, wings, tail stabilizers and fins, windshields, etc.). This build-up of ice is due to the fact that in flight the surface encounters super-cooled droplets of water in the atmosphere. This super-cooled state is a state of highly unstable equilibrium that can be disrupted if some very small quantity of energy is delivered to the water droplet, e.g. in the form of a mechanical impact. The water changes state and switches to the solid state. Thus, a wing or a blade passing through a zone of super-cooled rain gives all of the water droplets sufficient energy for them to change to the solid state. The aerodynamic structure then becomes covered in ice very quickly. This deposit of ice increases the weight of the aircraft, sometimes to a considerable extent, and interferes with the air flow by changing the shape of the aerodynamic surface, thereby greatly degrading its performance.
The drag coefficient CD and the lift coefficient CL of an airfoil are directly proportional to the shape of an aerodynamic surface and to its angle of incidence. The lift capacity of the aerodynamic surface is characterized by its efficiency, i.e. the fineness ratio which is the ratio of lift over drag. The efficiency of an airfoil increases with increasing ability to generate lift relative to drag. The formation of hard ice, soft ice, or frost leads to weight overload, increases drag CD of the aerodynamic surface, and reduces its lift CL. The operational performances of an aircraft are those affected directly by the phenomenon of icing.
The problem is often handled by fitting the aerodynamic surface with a Joule effect heater device. In general, a distinction is drawn between de-icers which are resistor elements that dissipate heat and that are powered intermittently in order to get rid of the ice that forms regularly, and anti-icers which are resistor elements that are powered continuously in order to prevent ice forming. Anti-icers are activated in preventative manner prior to entering icing conditions, whereas de-icers are used mainly in curative manner, once ice has already formed.
Document FR 2 578 377 discloses a Joule effect heater device made up of carbon fibers embedded in an organic matrix, generally of the epoxy type. Those composite resistor elements of conductive carbon fibers form tapes which preferably extend in the vicinity of the leading edge of an aerodynamic surface and in parallel therewith. In order to obtain Joule effect heating power that varies along the leading edge, a plurality of resistive layers are superposed.
Document FR 2 756 253 discloses a Joule effect heater device made of composite materials for the blades of a rotorcraft, the device comprising a plurality of tapes provided with at least one resistive layer of constant width and at least one resistive layer of continuously varying width. That configuration makes it possible to optimize variation in the electrical resistance of the device, in particular along a leading edge, so as to obtain the required heating power per unit area as specified by heat engineers during blade design.
Nevertheless, cutting out, laying, and assembling such resistive layers made of electrically-conductive carbon fibers impregnated in a thermosetting organic matrix raise several manufacturing problems because of the complexity involved. Each resistive layer is made of a set of longitudinal tapes. Two adjacent longitudinal tapes are interconnected by a transverse tape disposed perpendicularly at one of the ends of the two longitudinal tapes. As a result, electrical conduction between two adjacent longitudinal tapes is provided by a transverse tape.
That complex architecture requires a large amount of cutting-out to be performed which is economically penalizing.
Furthermore, the orientation of the longitudinal and transverse tapes must be accurately laid out, so the time required for manual draping of all the pieces turns out to be long.
Furthermore, the surface quality of the longitudinal and transverse tapes in a given resistive layer (contact area, local orientation of fibers), which is essential for proper operation of the resistive array as a whole, is not always effectively guaranteed. It can happen that dielectric plastic separators are forgotten during assembly resulting in the part being rejected. Similarly, transverse tapes sometimes slide from the manufacturing mold while the device is being shaped or while it is polymerizing.